The present invention relates generally to an insulating gate type semiconductor device, and more particularly to a trench gate type IEGT (Injection Enhanced Gate Transistor).
FIG. 1 is a plan view showing a trench gate type IEGT.
The trench gate type IEGT shown in FIG. 1 is structured such that a gate pad 52 and five element regions 54 are disposed within a terminal region 51 provided along a substrate peripheral region for obtaining a withstand voltage. Gate wires 53 are provided at boundaries of the terminal region 51, the gate pad 52 and the five element regions 54. The trench gate type IEGT has striped trench gate electrodes 6 extending as shown in FIG. 1 and disposed inwardly of each element region 54. A voltage applied to the gate pad 52 is transferred to the trench gate electrode 6 through the gate wire 53.
A conventional trench gate type IEGT will hereinafter be described.
FIG. 2 is a sectional view taken along the line A-Axe2x80x2 perpendicular to the trench gate electrode 6 of the trench gate type IEGT shown in FIG. 1, showing a structure of a first section of the prior art trench gate type IEGT.
The first section of the prior art trench gate type IEGT shown in FIG. 2 has a structure which follows. An N-type base layer 1 and a P-type base layer 3 are provided in sequence on a P-type emitter layer 2. Striped trenches for forming the trench gate electrode 6 are arranged substantially in parallel and each recessed extending from the substrate surface through the P-type base layer down to an upper portion of the N-type base layer 1, and a gate oxide layer 5 is provided over the substrate surface and a trench internal surface. A trench gate electrode 6 defined as a gate electrode composed of polysilicon with a resistance reduced is provided in an interior of the trench covered with the gate oxide layer 5.
Each set of trench gate electrodes 6 is constituted by fours arranged in sequence in the trench gate type IEGT in the first embodiment of the present invention. Among the four trench gate electrodes 6 constituting one set, the two electrode disposed at both side ends are classified as channel-forming trench gates 6a, and remaining two electrodes interposed therebetween are classified as thinning-out trench gates 6b. An N-type emitter layer 4 is provided in the vicinity of the channel forming trench gate electrode 6a as well as on the substrate surface of the P-type base layer 3 interposed between the channel forming trench gate electrode 6a belonging to one set of electrodes and the channel-forming trench gate electrode 6a belonging to another set of electrodes adjacent to the above one set of electrodes.
An insulating oxide layer 7 is formed covering some portions or the whole of the upper surfaces of the channel-forming trench gates 6a and the gate oxide layers 5 on the substrate surface. A contact hole 10 is holed in the insulating oxide layer 7 and the gate oxide layer 5 at the center of an inter-trench area formed with the N-type emitter layer 4. An emitter electrode 8 is provided over the entire surface of the substrate surface, covering all these layers and hole described above. Moreover, a collector electrode 9 is provided covering the entire surface of the P-type emitter layer 2 on the underside of the substrate. Accordingly, the emitter electrode 8 is connected at the contact hole 10 to the N-type emitter layer 4 and the P-type base layer 3.
In the trench gate type IEGT, the contact on the side of the emitter is not formed in all the inter-trench areas. In the case of the prior art trench gate type IEGT shown in FIG. 2, there is provided the contact region in which the single contact hole 10 is holed for every four inter-trench areas, and therefore a ratio of the total number of inter-trench areas to the number of the contact regions is 4:1.
The thinning-out trench gate electrode 6b, though not used for forming a channel, performs a role of preventing a decrease in element withstand voltage with such a contrivance that when in a forward voltage application, a depletion layer formed extending to a portion peripheral to a tip of the channel-forming trench gate electrode 6a is fused with a depletion layer formed extending to a portion peripheral to a tip of the thinning-out trench gate electrode 6b adjacent to the channel-forming trench gate electrode 6a, and a curvature of the depletion layer at the portion peripheral to the tip of the channel-forming trench gate electrode 6a is relieved. Accordingly, if the thinning-out trench gate electrode 6b is not provided, the element withstand voltage is to decrease.
FIG. 3 is a sectional view showing a structure of a second section of the trench gate type IEGT in the prior art, i.e., a sectional structure taken along the straight line B-Bxe2x80x2 intersecting a direction parallel to the trench gate electrode 6b with respect to the gate wire 53 of the trench gate type IEGT shown in FIG. 1.
The second section of the prior art trench gate type IEGT shown in FIG. 3 takes the following structure. The N-type base layer 1 is provided on the P-type emitter layer 2, and a P+ layer 13 is formed on the N-type base layer 1 at a portion under the gate wire 53 shown in FIG. 1. An insulating oxide layer 14 is provided on the gate wire region on the P+ layer 13. The trenches are formed on both sides of the P+ layer 13, and the gate oxide layer 5 is provided covering these components. The trench gate electrode 6 composed of the polysilicon is provided inwardly of the trench covered with the gate oxide layer 5. A trench gate drawing region 6xe2x80x2 composed of the polysilicon as in the same way with the trench gate electrode 6, is provided covering side ends of the trench gate electrode 6 on the side of the P+ layer 13, and an area of the gate oxide layer 5 which exists on the insulating oxide layer 14 and the P+ layer 13. The trench gate drawing region 6xe2x80x2 is connected to the side ends of the trench gate electrode 6 on the side of the P+ layer 13, whereby the trench gate electrode 6 is drawn out to the gate wire 53 and thus connected to the gate wire 53. The insulating oxide layer 7 is provided on the trench gate drawing region 6xe2x80x2, and the gate wire region of the insulating oxide layer 7 is removed. A gate-wire-oriented metal 15 for reducing a resistance is provided on the trench gate drawing region 6xe2x80x2 from above the insulating oxide layer 7 with the gate wire region removed. The gate wire 53 is constructed of the gate wire-oriented metal 15 and the trench gate drawing region 6xe2x80x2. The emitter electrode 8 is provided on a region of the insulating oxide layer 7, wherein the trench gate electrode 6 is formed.
FIG. 4 is a plan view showing, in a region C in FIG. 1, the prior art trench gate type IEGT. Note that FIG. 2 is a sectional structural view taken along the straight line A-Axe2x80x2 perpendicular to the trench gate electrodes 6 (6a and 6b ) shown in FIG. 4.
As illustrated in FIG. 4, the trench gate electrodes 6a, 6b are formed in a stripe shape alternately at a predetermined interval, and arranged in sequence by fours as one set. Among one set of four trench gate electrodes 6, two electrodes disposed at both side ends are classified as the channel forming trench gate electrodes 6a, and the remaining two electrodes interposed therebetween are classified as the thinning-out trench gate electrodes 6b. The insulating oxide layer 7 (not shown in FIG. 4) in FIG. 2 or 3 is formed covering some portions or the whole of the upper surfaces of the trench gates 6a, 6b. The contact hole 10 is holed in an area interposed between the channel-forming trench gate electrode 6a belonging to one set of electrodes and the channel-forming trench gate electrode 6a belonging to another set of electrodes adjacent to above one set of electrodes. The emitter electrode 8 (which is not shown in FIG. 4) in FIG. 2 or 3 is connected at the contact hole 10 to the N-type emitter layer 4 and the P-type base layer 3. Further, each of the channel-forming trench gate electrode 6a and the thinning-out trench gate electrode 6b is, with a structure of drawing the polysilicon out of side end thereof, connected to the gate wire 53.
In the prior art trench gate type IEGT shown in FIGS. 2 to 4, as explained above, there is adopted the contact thinning-out structure which involves providing the thinning-out trench gate electrode 6b. With this structure, a low forward ON-voltage is attained by increasing a hole discharge resistance and an electron implantation efficiency and when the element is in an ON-state (For further details of the prior art trench gate type IEGT, refer to xe2x80x9cA 4500V Injection Enhanced Insulated Gate Bipolar Transistor (IEGT) in a Mode Similar to a Thyristorxe2x80x9d, IEDM"" 93, pp. 679-682.xe2x80x99 by ""M. Kitagawa, I. Omura, S. Hasegawa, T. Inoue and A. Nakagawa).
In the prior art trench gate type IEGT described above, however, the thinning-out trench gate electrode is also connected to the gate pad, and a gate capacitance between the gate/collector electrode increases, resulting in a problem wherein large oscillations of waveforms of voltage and current when in switching are caused in the case of an element having a large area and operating the elements in parallel.
Accordingly, it is a primary object of the present invention to provide an insulating gate type semiconductor device capable of reducing voltage waveform oscillations of waveforms of voltage and current and an electric current when in switching in the case of a element having a large area and operating the elements in parallel by decreasing a gate capacitance without causing drawbacks such as a decline of manufacturing yield and a rise in gate wire resistance etc.
To accomplish the above object, according to one aspect of the present invention, an insulating gate type semiconductor device comprises a plurality of trench gate electrodes provided substantially in parallel, wherein among the trench gate electrodes, a thinning-out trench gate electrode excluding a channel-forming trench gate electrode is insulated from a gate wire and is connected to an emitter electrode or to a predetermined electric potential generating device for generating a negative electric potential with respect to an emitter potential. Namely, the thinning-out trench gate electrode is structured so that its electric potential is always equal or negative to an emitter potential. With this structure, it is possible to obtain the trench gate type IEGT capable of reducing the gate capacitance without inducing a drop in element withstand voltage and decreasing the oscillations of waveforms of voltage and current when in switching in the case lf the element having the large area and operating the elements in parallel.
According to another aspect of the present invention, an insulating gate type semiconductor device comprises a plurality of trench gate electrodes provided in a plurality of trenches arranged substantially in parallel and recessed at a first distance and a second distance alternately to a predetermined depth extending to an upper portion of an N-type base layer from the surface of a semiconductor substrate on which a P-type emitter layer, the N-type base layer and a P-type base layer are formed in sequence from the underside thereof to the surface thereof, wherein the trench gate electrodes are defined as channel-forming trench gate electrodes for forming a channel, one set of the trench gate electrodes is constituted by twos arranged in sequence, and the predetermined depth of the trench is set to such an extent that a depletion layer formed extending from a tip of the channel-forming trench gate electrode when in a forward voltage application is fused with a depletion layer formed extending from a junction area between the N-type base layer and the P-type base layer to which the trench gate electrode is vicinal and that a curvature of the depletion layer at the tip of the trench gate electrode is relieved.
The predetermined depth of the trench is such a depth that a depth from the junction surface between the N-type base layer and the P-type base layer is, to be specific, 3 xcexcm or under. With this configuration, the curvature of the depletion layer at the tip of the trench gate electrode is relieved, and hence, even if the thinning-out trench gate electrode is not provided, it never happens that the element withstand voltage decreases, with the result that the thinning-out trench gate electrode is not required. Accordingly, there is no necessity for a connection area between the thinning-out trench gate electrode and the emitter electrode. This might lead advantages in which the manufacturing yield is highly enhanced, and, in addition, there occurs no such problem that a crystal distortion is caused due to an internal stress of the trench with a reduced number of trenches within one element. Further, there is no thinning-out trench gate electrode disposed in close proximity to the channel-forming trench gate electrode, and consequently a gate capacitance between the gate and the emitter is reduced. It is feasible to obtain the trench gate type IEGT capable of, as will be demonstrated in the embodiments which follow, decreasing the oscillations of waveforms of voltage and current and the electric current when in switching in the case of the element having the large area and operating the elements in parallel.